In recent years, laser scanning techniques have been developed in the prior art in which information is transmitted to a scanned medium by modulating light from a laser in accordance with a signal, such as a video signal, which corresponds to the information to be reproduced on the scanned medium. For example, U.S. Pat. No. 3,867,571 discloses a flying spot scanning system which utilizes light from a multifaceted rotating polygon which is then directed to the scanned medium. A light source illuminates at least one of the facets during each scanning cycle to provide the spot scan. In each scanning cycle information is transmitted to the scanned medium by modulating the light from the light source in accordance with a video signal. In order to avoid the variation of the spot velocity at the focal plane of the spot due to the convolution of optical elements utilized in the system, the number of binary digits in the video signal transmitted to the modulator is varied. A function generator, whose output is the approximation of the predicted spot velocity throughout a scan line is coupled to a variable frequency clock generator which in turn is coupled to a digital device for varying the number of bits per second in accordance with a predetermined function to transmit the bit stream at a given rate synchronous with the velocity of the spot. The purpose is to control the video data rate which is applied to the modulator so as to be proportional to the spot velocity so that the resulting image on the scanned medium is not distorted.
The spot velocity correction technique described in the aforementioned patent is based upon velocity errors due to the optical components utilized and implies that the velocity error will be constant for each scanline. However, velocity variations in a scanning system of the type discribed can also be caused by errors in individual polygon facets which are differences in radius and angularity of the polygon from facet to facet which then may require a different correction signal for each scanning facet. U.S. Pat. No. 3,573,849 describes a technique for making the scan velocity of the writing beam across a photosensitive medium uniform by utilizing a scanning lens which has a varying focal length. This patent also utilizes a coding beam to maintain synchronism between the writing beam modulation and writing beam scan such that each information bit modulates the writing beam at the proper instant, with respect to the writing beam scan. However, the use of a coding plate to generate the coding beam and the use of the variable focal length scanning lens makes this technique relatively costly.
Therefore, what is desired is a facet error correction system for use in laser scanning applications which corrects for errors associated with each facet on the rotating polygon which is accurate and relatively inexpensive. One possible solution to the aforementioned problem is to use a polygon which is accurately machined in a manner where the facet errors are virtually eliminated. However, this would be extremely costly and would cost ineffective in products which may incorporate the laser scanning system as discussed above. The use of grating clock type systems as, for example, disclosed in the aforementioned U.S. Pat. No. 3,573,849, is also relatively costly.